Current CAT scan systems usually include a gantry formed of a structure such as a centrally apertured disk or drum rotatable within a frame, and at least an X-ray source mounted on the drum for rotational motion about a table on which a patient can repose. The X-ray source may provide periodic pulses or continuous wave radiation. In third generation CAT scan systems the X-ray detector system is in the form of a detector array secured to the drum, diametrically opposite the source. In fourth generation CAT scan systems the X-ray detectors are disposed around the frame and are positioned to detect X-rays as the drum rotates about its axis. Each detector typically is either solid state or a gas tube. The detector system is aligned with the source so that the detector system and source are positioned within a common mean, scanning or rotation plane (usually perpendicular to the axis of rotation of the drum). In the case of a detector array found on third generation systems, each detector of the detector array is positioned in the scanning plane at a predetermined angular spacing relative to the source so that each detector subtends an equal angle relative to the focal spot of the X-ray tube, thus providing a plurality of different X-ray paths in the scanning plane between the source and the respective detectors. In fourth generation machines the stationary detectors each define a focal point for the X-ray paths from various positions of the source, as the latter rotates about the rotation axis. In both types of systems the X-ray paths can collectively resemble a fan, and consequently such systems are sometimes referred to as "fan beam" tomography systems.
These tomography systems each provides a plurality of information or data signals representing variations in the radiation flux measured by the respective detectors at predetermined angular positions of the drum during rotation of the drum about an object positioned in the space between the detectors and the X-ray source. Processing these data signals in accordance with known (Radon) mathematical relationships, a visual image can be formed, or "reconstructed," representing a two-dimensional slice along the plane of rotation, i.e., the scanning plane, through the portion of the scanned object positioned in the plane. The formation of such "reconstructed" images critically depends upon the components mounted on the rotating drum rotating precisely about the axis of rotation so that no lateral movement occurs between the moving drum relative to the object being scanned.
Because even minor mechanical noise and/or artifacts causing undesirable lateral motion of the elements of the CAT scan apparatus during a scan relative to the object, particularly those occurring within the scanning plane normal to the axis of rotation, can cause errors resulting in faulty or erroneous image information, such apparatus typically has been provided as massively reinforced machines often weighing a ton or more in order to reduce motion due to such mechanical noise and artifacts. Consequently, because of the weight, the massive drum has usually been supported in the frame by an expensive and heavy precision roller bearing or ball bearing assembly.
Many of the disadvantages inherent in such a massive, expensive, relatively fixed CAT scan structure characteristic of the prior art have been recognized and addressed, at least in part, by the apparatus described and claimed in U.S. Pat. No. 4,928,283 issued May 22, 1990 to B. M. Gordon, and in U.S. Pat. No. 5,109,397 issued Apr. 28, 1992 to B. M. Gordon, et al., both assigned to the present assignee. In the aforesaid '283 patent, the patentee broadly suggests the use of wheels rather than bearings for rotatably supporting the drum in a frame, without however any discussion of the nature and characteristics of such wheels. The simple replacement of bearings with wheels may introduce deviations or wobble as the drum is rotated in its plane, resulting in undesirable inaccuracies in the tomographic image produced. The '397 patent addresses, inter alia, the use of electromechanical sensors that follow the outer periphery of the disc in the plane of rotation to provide compensating electrical signals for modifying or correcting the data received by the X-ray detection array.
In addition, the massive drum of the prior art system can be subjected to changes in temperature from heat generated by the X-ray source. To the extent this heat is transferred to the drum, the drum will expand in accordance with its coefficient of thermal expansion, and only contract when the source is allowed to cool. Since the drum is sized to rotate within the bearing race at ambient temperature, as the drum expands compressive forces are created on the bearings resulting in stress related failures. Accordingly, heat transfer devices and the like are frequently used so as to minimize the effects of heat on the components from the use of the X-ray source. However, heat sinks merely add additional weight and therefore stress to the bearings. As a result, the bearings and races have to be replaced from time to time. But because of the weight of the drum, this problem can be difficult to service and time consuming. While wheels are suggested in the '283 patent as a replacement for the bearings, the patent is silent as to the problem associated with heat expansion or how wheels overcome the problem.